1. Field of the Invention
The present invention relates to power supply apparatuses for regulating a voltage generated by a DC power supply and outputting the regulated voltage.
2. Description of the Related Art
Recently, many electronic apparatuses that use dry cells, such as nickel-cadmium batteries and nickel-metal-hydride batteries, as the power supply source have become generally used. Since these dry cells have relatively low output voltage ranges, the voltage that can be supplied by the dry cell does not necessarily match the voltage used by the electronic apparatus. Thus, the output voltage of the dry cell is converted by a voltage transducer, which is referred to as a DC/DC converter. As a result, the power supply voltage can be stably supplied to an electronic circuit.
FIG. 8 shows an example of the configuration of a known step-up voltage transducer circuit.
A voltage transducer circuit 100 shown in FIG. 8 is a step-up converter for boosting an input voltage and outputting the boosted voltage. In the voltage transducer circuit 100, the drain of a transistor Q21 is connected via a choke coil L21 to a power supply terminal 12a, and the source is grounded. A pulse input terminal 12b for receiving a switching pulse from an oscillation circuit (PWM: Pulse Width Modulator) (not shown) is connected to the gate. The anode of a diode (Schottky diode) D21 is connected to the node between the transistor Q21 and the choke coil L21. A capacitor C21 is connected to the cathode of the diode D21. The other end of the capacitor C21 is grounded. An output terminal 12c (to a load) and a feedback terminal 12d (to an error amplifier) are connected to the node between the diode D21 and the capacitor C21.
The transistor Q21 is an n-channel MOS-FET (Metal Oxide Semiconductor-Field Effect Transistor). In accordance with a switching pulse from the pulse input terminal 12b, the transistor Q21 enters an ON or OFF state, thereby functioning as a switching element. When the transistor Q21 is changed from ON to OFF in response to a switching pulse, energy excited by the choke coil L21 is released to the node between the choke coil L21 and the diode D21. As a result, a voltage higher than that of the power supply terminal 12a is generated, and the capacitor C21 is charged. Subsequently, the voltage at the node gradually decreases. When the transistor Q21 is turned ON the next time, the voltage at the node becomes substantially equal with the ground voltage. Accordingly, a voltage fluctuation in response to the switching pulse occurs at the anode of the diode D21. The fluctuation is rectified by the diode D21, thus generating a voltage higher than the input voltage. The voltage is smoothed by the capacitor C21, the smoothed voltage is removed from the output terminal 12c, and the voltage is supplied to the load.
The feedback terminal 12d outputs the same voltage as that of the output terminal 12c, and the output voltage is supplied to the error amplifier (not shown). At the error amplifier, the voltage output from the feedback terminal 12d is compared with a predetermined voltage. In accordance with the comparison signal, a switching pulse output from the oscillation circuit is controlled.
In many cases, a power supply apparatus using a dry cell generally has an output voltage of around 1 V. On the other hand, when a voltage less than or equal to approximately 1 V is input to a power supply apparatus using a known voltage transducer circuit, such as the foregoing voltage transducer circuit 100, the power supply apparatus having a relatively large load cannot be activated. Even if the power supply apparatus can be activated, the subsequent operation may become unstable. FIG. 9 is a graph showing an example of the relationship between load power and starting voltage of a known power supply circuit.
FIG. 9 shows the relationship of load power with a starting voltage (Vstart) of the known power supply circuit and the minimum allowable input voltage (Vhold) for stably operating the power supply circuit. According to FIG. 9, when the load power is less than or equal to approximately 25 mW, the power supply circuit can be activated by an input voltage ranging from approximately 0.8 to 1.0 V. As the load power increases, the starting voltage also increases. When the load power is greater than or equal to 25 mW, the starting voltage increases substantially in proportion to the load power.
According to FIG. 9, if the input voltage slightly decreases relative to the starting voltage subsequent to activation, the power supply circuit can be normally operated. When the load power is less than or equal to approximately 15 mW, the minimum allowable range increases as the load power decreases. When the load power is greater than or equal to 15 mW, the minimum allowable range is smaller and changes at an approximately constant rate.
When the voltage input to the known power supply apparatus slightly falls below 1 V, if the load power is approximately 25 mW, the known power supply apparatus can be activated and operated normally. If the load power is greater than 25 mW, the known power supply apparatus cannot be activated. When the input voltage decreases after activation, the operation of the known power supply apparatus may become unstable since the range in which the power supply apparatus can be normally operated is small. These problems may be caused by the fact that the oscillation circuit for outputting a switching pulse cannot oscillate normally due to a low voltage or the fact that normal switching operation for the choke coil cannot be performed since the gate voltage of a switching element (FET) in the voltage transducer circuit is too small.
In a power supply apparatus which uses the voltage transducer circuit 100 arranged as shown in FIG. 8 and which supplies a constant voltage generated by boosting an input voltage to a load, if the input voltage is less than a predetermined voltage, it is necessary to increase the step-up ratio. If Ton represents time during which the transistor Q21 in the voltage transducer circuit 100 is turned ON and Toff represents time during which the transistor Q21 is turned OFF, the relationship between the input voltage Vin from the power supply terminal 12a and the output voltage Vout from the output terminal 12c can be represented as:                     Vout        =                                            (                              Ton                +                Toff                            )                        xc3x97            Vin                    Toff                                    (        1        )            
When a dry cell is used as the power supply source for the voltage transducer circuit 100, Vin may be a value around 1 V. If, for example, Vin is 1 V and Vout is 5 V, then the ratio Ton:Toff=4:1, and the duty ratio in the Ton period is 80%. In this case, if the input voltage becomes smaller than 1 V, the duty ratio may be further biased. When the load is large, activation may become difficult, and the stability during the normal operation may deteriorate.
In view of the foregoing problems, it is an object of the present invention to provide a power supply apparatus capable of stably operating while having a high load even when an input voltage is low.
According to the present invention, the foregoing objects are achieved through provision of a power supply apparatus for regulating a voltage generated by a DC power supply and for outputting the voltage to a predetermined load. The power supply apparatus includes a first voltage transducer for boosting the voltage generated by the DC power supply; a second voltage transducer for converting the voltage generated by the DC power supply to a predetermined voltage and for outputting the converted voltage to the predetermined load; an operation controller driven by an output voltage from the first voltage transducer, the operation controller outputting a switching pulse in accordance with reception of an activation signal and controlling the operation of the second voltage transducer; and an activation controller for inputting the output voltage from the first voltage transducer to the operation controller and, after a predetermined period of time, inputting the activation signal to the operation controller.
In the power supply apparatus, the voltage generated by the DC power supply is boosted by the first voltage transducer, and the boosted voltage is supplied to the operation control circuit for the second voltage transducer. Under the control of the activation controller, after a predetermined period of time has passed since the output voltage from the first voltage transducer was started to be supplied to the operation controller, the operation controller receives the activation signal. Accordingly, the operation control circuit outputs the switching pulse, thus starts controlling the operation of the second voltage transducer driven by the output voltage from the DC power supply. For example, the second voltage transducer includes the charge pump circuit at the output stage.
Accordingly, when the output voltage of the DC power supply decreases under a heavy load, the operation controller can be reliably activated by a voltage boosted by the first voltage transducer, and hence the operation controller can control the second voltage transducer in a stable manner. Since, for example, the second voltage transducer includes the charge pump circuit at the output stage, in addition to the stable operation of the operation control circuit, even if the step-up ratio is high, the output voltage of the DC power supply is not reduced, and the second voltage transducer can reliably output a voltage to the heavy load.